Self resonant transmitting device

ABSTRACT

A device for powering an implant within a body of a subject from a location external to the subject, wherein the implant requires a threshold rate of power increase in order to operate in at least one mode, may include an antenna configured to wirelessly transmit energy to the implant. The device may also include a power storage unit configured to store energy from a power source incapable of delivering the threshold rate of power increase to enable the implant unit to operate in the at least one mode and a power release unit configured to release a pulse of energy from the power storage unit to the antenna after the power storage unit collects an amount of energy sufficient to enable the implant unit to operate in the at least one mode.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 13/952,015,filed on Jul. 26, 2013, which claims the benefit of priority under 35U.S.C. §119(e) to U.S. Provisional Application No. 61/676,327, filedJul. 26, 2012, each of which are incorporated herein by reference.

TECHNICAL FIELD

Embodiments of the present disclosure generally relate to devices andmethods for conveying power from a location external to a subject to alocation within the subject. More particularly, embodiments of thepresent disclosure relate to devices and methods for transcutaneouslyconveying power to an implanted neuromodulation device.

BACKGROUND

Neural modulation presents the opportunity to treat many physiologicalconditions and disorders by interacting with the body's own naturalneural processes. Neural modulation includes inhibition (e.g. blockage),stimulation, modification, regulation, or therapeutic alteration ofactivity, electrical or chemical, in the central, peripheral, orautonomic nervous system. By modulating the activity of the nervoussystem, for example through the stimulation of nerves or the blockage ofnerve signals, several different goals may be achieved. Motor neuronsmay be stimulated at appropriate times to cause muscle contractions.Sensory neurons may be blocked, for instance to relieve pain, orstimulated, for instance to provide a signal to a subject. In otherexamples, modulation of the autonomic nervous system may be used toadjust various involuntary physiological parameters, such as heart rateand blood pressure. Neural modulation may provide the opportunity totreat several diseases or physiological conditions, a few examples ofwhich are described in detail below.

Among the conditions to which neural modulation may be applied is sleepdisordered breathing, examples of which include obstructive sleep apnea(OSA) and snoring. OSA is a respiratory disorder characterized byrecurrent episodes of partial or complete obstruction of the upperairway during sleep. During the sleep of a person without OSA, thepharyngeal muscles relax during sleep and gradually collapse, narrowingthe airway. The airway narrowing limits the effectiveness of thesleeper's breathing, causing a rise in CO₂ levels in the blood. Theincrease in CO₂ results in the pharyngeal muscles contracting to openthe airway to restore proper breathing. The largest of the pharyngealmuscles responsible for upper airway dilation is the genioglossusmuscle, which is one of several different muscles in the tongue. Thegenioglossus muscle is responsible for forward tongue movement and thestiffening of the anterior pharyngeal wall. In patients with OSA, theneuromuscular activity of the genioglossus muscle is decreased comparedto normal individuals, accounting for insufficient response andcontraction to open the airway as compared to a normal individual. Thislack of response contributes to a partial or total airway obstruction,which significantly limits the effectiveness of the sleeper's breathing.In OSA patients, there are often several airway obstruction eventsduring the night. Because of the obstruction, there is a gradualdecrease of oxygen levels in the blood (hypoxemia). Hypoxemia leads tonight time arousals, which may be registered by EEG, showing that thebrain awakes from any stage of sleep to a short arousal. During thearousal, there is a conscious breath or gasp, which resolves the airwayobstruction. An increase in sympathetic tone activity rate through therelease of hormones such as epinephrine and noradrenaline also oftenoccurs as a response to hypoxemia. As a result of the increase insympathetic tone, the heart enlarges in an attempt to pump more bloodand increase the blood pressure and heart rate, further arousing thepatient. After the resolution of the apnea event, as the patient returnsto sleep, the airway collapses again, leading to further arousals.

These repeated arousals, combined with repeated hypoxemia, leaves thepatient sleep deprived, which leads to daytime somnolence and worsenscognitive function. This cycle can repeat itself up to hundreds of timesper night in severe patients. Thus, the repeated fluctuations in andsympathetic tone and episodes of elevated blood pressure during thenight evolve to high blood pressure through the entire day.Subsequently, high blood pressure and increased heart rate may causeother diseases.

Efforts for treating OSA include Continuous Positive Airway Pressure(CPAP) treatment, which requires the patient to wear a mask throughwhich air is blown into the nostrils to keep the airway open. Othertreatment options include the implantation of rigid inserts in the softpalate to provide structural support, tracheotomies, or tissue ablation.

Another condition to which neural modulation may be applied is theoccurrence of migraine headaches. Pain sensation in the head istransmitted to the brain via the occipital nerve, specifically thegreater occipital nerve, and the trigeminal nerve. When a subjectexperiences head pain, such as during a migraine headache, theinhibition of these nerves may serve to decrease or eliminate thesensation of pain.

Neural modulation may also be applied to hypertension. Blood pressure inthe body is controlled via multiple feedback mechanisms. For example,baroreceptors in the carotid body in the carotid artery are sensitive toblood pressure changes within the carotid artery. The baroreceptorsgenerate signals that are conducted to the brain via theglossopharyngeal nerve when blood pressure rises, signaling the brain toactivate the body's regulation system to lower blood pressure, e.g.through changes to heart rate, and vasodilation/vasoconstriction.Conversely, parasympathetic nerve fibers on and around the renalarteries generate signals that are carried to the kidneys to initiateactions, such as salt retention and the release of angiotensin, whichraise blood pressure. Modulating these nerves may provide the ability toexert some external control over blood pressure.

The foregoing are just a few examples of conditions to whichneuromodulation may be of benefit, however embodiments of the inventiondescribed hereafter are not necessarily limited to treating only theabove-described conditions.

SUMMARY

A device according to some embodiments of the disclosure may beconfigured for powering an implant within a body of a subject from alocation external to the subject, wherein the implant requires athreshold rate of power increase in order to operate in at least onemode. Such a device may include an antenna configured to wirelesslytransmit energy to the implant, a power storage unit configured to storeenergy from a power source incapable of delivering the threshold rate ofpower increase to enable the implant unit to operate in the at least onemode, and a power release unit configured to release a pulse of energyfrom the power storage unit to the antenna after the power storage unitcollects an amount of energy sufficient to enable the implant unit tooperate in the at least one mode.

In another embodiment, a device for transmitting power from a locationexternal to a body of a subject to an implant unit internal to the bodyof the subject may be provided. The device may include an externalcircuit including an antenna and having a resonant frequency that variestemporally based on, e.g., environmental conditions, and a processorconfigured to cause delivery of an impulse of energy to the externalcircuit to excite the external circuit and cause oscillations in theexternal circuit at a current resonant frequency of the external circuitsuch that power is delivered from the antenna to the implant unit at thecurrent resonant frequency of the external circuit

In still another embodiment, a device for transmitting power from alocation external to a body of a subject to an implant unit internal tothe body of the subject is provided. The device may include an antennaconfigured to be located external to a subject, a power source, a powerstorage unit, a power release unit, and at least one processorconfigured to control the power release unit. In the device, at leastthe antenna may be part of an external circuit having a resonancefrequency. Furthermore, the power release unit may be configured suchthat, when in an open position, the power storage unit receives powerfrom the primary power source and, when in a closed position, the powerrelease unit releases power to the primary antenna, generating anoscillation in the primary antenna at the resonance frequency.

Additional features of the disclosure will be set forth in part in thedescription that follows, and in part will be obvious from thedescription, or may be learned by practice of the disclosed embodiments.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory only,and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of thedisclosure and, together with the description, serve to explain theprinciples of the embodiments disclosed herein.

FIG. 1 diagrammatically illustrates an implant unit and external unit,according to an exemplary embodiment of the present disclosure.

FIG. 2 is a partially cross-sectioned side view of a subject with animplant unit and external unit, according to an exemplary embodiment ofthe present disclosure.

FIG. 3 diagrammatically illustrates a system including an implant unitand an external unit, according to an exemplary embodiment of thepresent disclosure.

FIG. 4 is a top view of an implant unit, according to an exemplaryembodiment of the present disclosure.

FIG. 5 is a top view of an another embodiment of an implant unit,according to an exemplary embodiment of the present disclosure.

FIG. 6 illustrates circuitry of an implant unit and an external unit,according to an exemplary embodiment of the present disclosure.

FIG. 7 depicts a self-resonant transmitter employing a modified class Damplifier.

FIG. 8 depicts a pulsed mode self-resonant transmitter.

FIG. 9 illustrates a graph of quantities that may be used in determiningenergy delivery as a function coupling, according to an exemplarydisclosed embodiment.

FIGS. 10 a and 10 b illustrate a double-layer crossover antennaaccording to exemplary embodiments of the present disclosure.

FIGS. 11 a and 11 b illustrate an exemplary embodiment of an externalunit.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Reference will now be made in detail to exemplary embodiments of thepresent disclosure, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

Embodiments of the present disclosure relate generally to a device formodulating a nerve through the delivery of energy. Nerve modulation, orneural modulation, includes inhibition (e.g. blockage), stimulation,modification, regulation, or therapeutic alteration of activity,electrical or chemical, in the central, peripheral, or autonomic nervoussystem. Nerve modulation may take the form of nerve stimulation, whichmay include providing energy to the nerve to create a voltage changesufficient for the nerve to activate, or propagate an electrical signalof its own. Nerve modulation may also take the form of nerve inhibition,which may including providing energy to the nerve sufficient to preventthe nerve from propagating electrical signals. Nerve inhibition may beperformed through the constant application of energy, and may also beperformed through the application of enough energy to inhibit thefunction of the nerve for some time after the application. Other formsof neural modulation may modify the function of a nerve, causing aheightened or lessened degree of sensitivity. As referred to herein,modulation of a nerve may include modulation of an entire nerve and/ormodulation of a portion of a nerve. For example, modulation of a motorneuron may be performed to affect only those portions of the neuron thatare distal of the location to which energy is applied.

In patients with OSA, for example, a primary target response of nervestimulation may include contraction of a tongue muscle (e.g., themuscle) in order to move the tongue to a position that does not blockthe patient's airway. In the treatment of migraine headaches, nerveinhibition may be used to reduce or eliminate the sensation of pain. Inthe treatment of hypertension, neural modulation may be used toincrease, decrease, eliminate or otherwise modify nerve signalsgenerated by the body to regulate blood pressure.

While embodiments of the present disclosure may be disclosed for use inpatients with specific conditions, the embodiments may be used inconjunction with any patient/portion of a body where nerve modulationmay be desired. That is, in addition to use in patients with OSA,migraine headaches, or hypertension, embodiments of the presentdisclosure may be use in many other areas, including, but not limitedto: deep brain stimulation (e.g., treatment of epilepsy, Parkinson's,and depression); cardiac pace-making, stomach muscle stimulation (e.g.,treatment of obesity), back pain, incontinence, menstrual pain, and/orany other condition that may be affected by neural modulation.

FIG. 1 illustrates an implant unit and external unit, according to anexemplary embodiment of the present disclosure. An implant unit 110, maybe configured for implantation in a subject, in a location that permitsit to modulate a nerve 115. The implant unit 110 may be located in asubject such that intervening tissue 111 exists between the implant unit110 and the nerve 115. Intervening tissue may include muscle tissue,connective tissue, organ tissue, or any other type of biological tissue.Thus, location of implant unit 110 does not require contact with nerve115 for effective neuromodulation. The implant unit 110 may also belocated directly adjacent to nerve 115, such that no intervening tissue111 exists.

In treating OSA, implant unit 110 may be located on a genioglossusmuscle of a patient. Such a location is suitable for modulation of thehypoglossal nerve, branches of which run inside the genioglossus muscle.Implant unit 110 may also be configured for placement in otherlocations. For example, migraine treatment may require subcutaneousimplantation in the back of the neck, near the hairline of a subject, orbehind the ear of a subject, to modulate the greater occipital nerveand/or the trigeminal nerve. Treating hypertension may require theimplantation of a neuromodulation implant intravascularly inside therenal artery or renal vein (to modulate the parasympathetic renalnerves), either unilaterally or bilaterally, inside the carotid arteryor jugular vein (to modulate the glossopharyngeal nerve through thecarotid baroreceptors). Alternatively or additionally, treatinghypertension may require the implantation of a neuromodulation implantsubcutaneously, behind the ear or in the neck, for example, to directlymodulate the glossopharyngeal nerve.

External unit 120 may be configured for location external to a patient,either directly contacting, or close to the skin 112 of the patient.External unit 120 may be configured to be affixed to the patient, forexample, by adhering to the skin 112 of the patient, or through a bandor other device configured to hold external unit 120 in place. Adherenceto the skin of external unit 120 may occur such that it is in thevicinity of the location of implant unit 110.

FIG. 2 illustrates an exemplary embodiment of a neuromodulation systemfor delivering energy in a patient 100 with OSA. The system may includean external unit 120 that may be configured for location external to thepatient. As illustrated in FIG. 2, external unit 120 may be configuredto be affixed to the patient 100. FIG. 2 illustrates that in a patient100 with OSA, the external unit 120 may be configured for placementunderneath the patient's chin (e.g., in a location where the externalunit in on a side of the patient's skin opposite to a location ofterminal fibers of the hypoglossal nerve) and/or on the front ofpatient's neck. The suitability of placement locations may be determinedby communication between external unit 120 and implant unit 110,discussed in greater detail below. In alternate embodiments, for thetreatment of conditions other than OSA, the external unit may beconfigured to be affixed anywhere suitable on a patient, such as theback of a patient's neck, i.e. for communication with a migrainetreatment implant unit, on the outer portion of a patient's abdomen,i.e. for communication with a stomach modulating implant unit, on apatient's back, i.e. for communication with a renal artery modulatingimplant unit, and/or on any other suitable external location on apatient's skin, depending on the requirements of a particularapplication.

External unit 120 may further be configured to be affixed to analternative location proximate to the patient. For example, in oneembodiment, the external unit may be configured to fixedly or removablyadhere to a strap or a band that may be configured to wrap around a partof a patient's body. Alternatively, or in addition, the external unitmay be configured to remain in a desired location external to thepatient's body without adhering to that location.

The external unit 120 may include a housing. The housing may include anysuitable container configured for retaining components. In addition,while the external unit is illustrated schematically in FIG. 2, thehousing may be any suitable size and/or shape and may be rigid orflexible. Non-limiting examples of housings for the external unit 100include one or more of patches, buttons, or other receptacles havingvarying shapes and dimensions and constructed of any suitable material.In one embodiment, for example, the housing may include a flexiblematerial such that the external unit may be configured to conform to adesired location. For example, as illustrated in FIG. 2, the externalunit may include a skin patch, which, in turn, may include a flexiblesubstrate. The material of the flexible substrate may include, but isnot limited to, plastic, silicone, woven natural fibers, and othersuitable polymers, copolymers, and combinations thereof. Any portion ofexternal unit 120 may be flexible or rigid, depending on therequirements of a particular application.

As previously discussed, in some embodiments external unit 120 may beconfigured to adhere to a desired location. Accordingly, in someembodiments, at least one side of the housing may include an adhesivematerial. The adhesive material may include a biocompatible material andmay allow for a patient to adhere the external unit to the desiredlocation and remove the external unit upon completion of use. Theadhesive may be configured for single or multiple uses of the externalunit. Suitable adhesive materials may include, but are not limited tobiocompatible glues, starches, elastomers, thermoplastics, andemulsions.

FIG. 3 schematically illustrates a system including external unit 120and an implant unit 110. In some embodiments, internal unit 110 may beconfigured as a unit to be implanted into the body of a patient, andexternal unit 120 may be configured to send signals to and/or receivesignals from implant unit 110.

As shown in FIG. 3, various components may be included within a housingof external unit 120 or otherwise associated with external unit 120. Asillustrated in FIG. 3, at least one processor 144 may be associated withexternal unit 120. For example, the at least one processor 144 may belocated within the housing of external unit 120. In alternativeembodiments, the at least one processor may be configured for wired orwireless communication with the external unit from a location externalto the housing.

The at least one processor may include any electric circuit that may beconfigured to perform a logic operation on at least one input variable.The at least one processor may therefore include one or more integratedcircuits, microchips, microcontrollers, and microprocessors, which maybe all or part of a central processing unit (CPU), a digital signalprocessor (DSP), a field programmable gate array (FPGA), or any othercircuit known to those skilled in the art that may be suitable forexecuting instructions or performing logic operations.

FIG. 3 illustrates that the external unit 120 may further be associatedwith a power source 140. The power source may be removably coupled tothe external unit at an exterior location relative to external unit.Alternatively, as shown in FIG. 3, power source 140 may be permanentlyor removably coupled to a location within external unit 120. The powersource may further include any suitable source of power configured to bein electrical communication with the processor. In one embodiment, forexample the power source 140 may include a battery.

The power source may be configured to power various components withinthe external unit. As illustrated in FIG. 3, power source 140 may beconfigured to provide power to the processor 144. In addition, the powersource 140 may be configured to provide power to a signal source 142.The signal source 142 may be in communication with the processor 144 andmay include any device configured to generate a signal (e.g., asinusoidal signal, square wave, triangle wave, microwave,radio-frequency (RF) signal, or any other type of electromagneticsignal). Signal source 142 may include, but is not limited to, awaveform generator that may be configured to generate alternatingcurrent (AC) signals and/or direct current (DC) signals. In oneembodiment, for example, signal source 142 may be configured to generatean AC signal for transmission to one or more other components. Signalsource 142 may be configured to generate a signal of any suitablefrequency. In some embodiments, signal source 142 may be configured togenerate a signal having a frequency of from about 6.5 MHz to about 13.6MHz. In additional embodiments, signal source 142 may be configured togenerate a signal having a frequency of from about 7.4 to about 8.8 MHz.In further embodiments, signal source 142 may generate a signal having afrequency as low as 90 kHz or as high as 28 MHz.

Signal source 142 may be configured for direct or indirect electricalcommunication with an amplifier 146. The amplifier may include anysuitable device configured to amplify one or more signals generated fromsignal source 142. Amplifier 146 may include one or more of varioustypes of amplification devices, including, for example, transistor baseddevices, operational amplifiers, RF amplifiers, power amplifiers, or anyother type of device that can increase the gain associated one or moreaspects of a signal. The amplifier may further be configured to outputthe amplified signals to one or more components within external unit120.

The external unit may additionally include a primary antenna 150. Theprimary antenna may be configured as part of a circuit within externalunit 120 and may be coupled either directly or indirectly to variouscomponents in external unit 120. For example, as shown in FIG. 3,primary antenna 150 may be configured for communication with theamplifier 146.

The primary antenna may include any conductive structure that may beconfigured to create an electromagnetic field. The primary antenna mayfurther be of any suitable size, shape, and/or configuration. The size,shape, and/or configuration may be determined by the size of thepatient, the placement location of the implant unit, the size and/orshape of the implant unit, the amount of energy required to modulate anerve, a location of a nerve to be modulated, the type of receivingelectronics present on the implant unit, etc. The primary antenna mayinclude any suitable antenna known to those skilled in the art that maybe configured to send and/or receive signals. Suitable antennas mayinclude, but are not limited to, a long-wire antenna, a patch antenna, ahelical antenna, etc. In one embodiment, for example, as illustrated inFIG. 3, primary antenna 150 may include a coil antenna. Such a coilantenna may be made from any suitable conductive material and may beconfigured to include any suitable arrangement of conductive coils(e.g., diameter, number of coils, layout of coils, etc.). A coil antennasuitable for use as primary antenna 150 may have a diameter of betweenabout 1 cm and 10 cm, and may be circular or oval shaped. In someembodiments, a coil antenna may have a diameter between 5 cm and 7 cm,and may be oval shaped. A coil antenna suitable for use as primaryantenna 150 may have any number of windings, e.g. 4, 8, 12, or more. Acoil antenna suitable for use as primary antenna 150 may have a wirediameter between about 0.1 mm and 2 mm. These antenna parameters areexemplary only, and may be adjusted above or below the ranges given toachieve suitable results.

As noted, implant unit 110 may be configured to be implanted in apatient's body (e.g., beneath the patient's skin). FIG. 2 illustratesthat the implant unit 110 may be configured to be implanted formodulation of a nerve associated with a muscle of the subject's tongue130. For example, implant unit 110 may be configured to modulate ahypoglossal nerve of a subject, e.g., through interaction with terminalfibers of the hypoglossal nerve. Modulating a nerve associated with amuscle of the subject's tongue 130 may include stimulation to cause amuscle contraction. In further embodiments, the implant unit may beconfigured to be placed in conjunction with any nerve that one maydesire to modulate. For example, modulation of the occipital nerve, thegreater occipital nerve, and/or the trigeminal nerve may be useful fortreating pain sensation in the head, such as that from migraines.Modulation of parasympathetic nerve fibers on and around the renalarteries (i.e. the renal nerves), the vagus nerve, and/or theglossopharyngeal nerve may be useful for treating hypertension.Additionally, any nerve of the peripheral nervous system (both spinaland cranial), including motor neurons, sensory neurons, sympatheticneurons and parasympathetic neurons, may be modulated to achieve adesired effect.

Implant unit 110 may be formed of any materials suitable forimplantation into the body of a patient. In some embodiments, implantunit 110 may include a flexible carrier 161 (FIG. 4) including aflexible, biocompatible material. Such materials may include, forexample, silicone, polyimides, phenyltrimethoxysilane (PTMS), polymethylmethacrylate (PMMA), Parylene C, polyimide, liquid polyimide, laminatedpolyimide, black epoxy, polyether ether ketone (PEEK), Liquid CrystalPolymer (LCP), Kapton, etc. Implant unit 110 may further includecircuitry including conductive materials, such as gold, platinum,titanium, or any other biocompatible conductive material or combinationof materials. Implant unit 110 and flexible carrier 161 may also befabricated with a thickness suitable for implantation under a patient'sskin. Implant 110 may have thickness of less than about 4 mm or lessthan about 2 mm.

Other components that may be included in or otherwise associated withthe implant unit are illustrated in FIG. 3. For example, implant unit110 may include a secondary antenna 152 mounted onto or integrated withflexible carrier 161. Similar to the primary antenna, the secondaryantenna may include any suitable antenna known to those skilled in theart that may be configured to send and/or receive signals. The secondaryantenna may include any suitable size, shape, and/or configuration. Thesize, shape and/or configuration may be determined by the size of thepatient, the placement location of the implant unit, the amount ofenergy required to modulate the nerve, etc. Suitable antennas mayinclude, but are not limited to, a long-wire antenna, a patch antenna, ahelical antenna, etc. In some embodiments, for example, secondaryantenna 152 may include a coil antenna having a circular shape (see alsoFIG. 4) or oval shape. Such a coil antenna may be made from any suitableconductive material and may be configured to include any suitablearrangement of conductive coils (e.g., diameter, number of coils, layoutof coils, etc.). A coil antenna suitable for use as secondary antenna152 may have a diameter of between about 5 mm and 30 mm, and may becircular or oval shaped. A coil antenna suitable for use as secondaryantenna 152 may have any number of windings, e.g. 4, 15, 20, 30, or 50.A coil antenna suitable for use as secondary antenna 152 may have a wirediameter between about 0.01 mm and 1 mm. These antenna parameters areexemplary only, and may be adjusted above or below the ranges given toachieve suitable results.

FIGS. 10 a and 10 b illustrate an exemplary double-layer crossoverantenna 1101 suitable for use as either primary antenna 150 or secondaryantenna 152. While a double-layer crossover antenna is shown anddescribed, other antenna configurations may be suitable for primaryantenna 150 and/or secondary antenna 152. For example, single layerantennas may be used where antenna components (e.g., coils) are arrangedin a single layer, e.g., either on or within a dielectric or insulatingmaterial. Also, while a crossover pattern is shown, other patterns mayalso be suitable. For example, in some embodiments, a wire associatedwith primary antenna 150 and/or secondary antenna 152 may include apattern of traces of progressively decreasing dimension. In the case oftraces arranged in coils, for example, each loop could include rings ofprogressively decreasing diameter to create a pattern that spiralsinwardly. A similar approach may be viable using traces of other shapesas well.

Returning to FIG. 10 a, this figure illustrates a single coil ofdouble-layer crossover antenna 1101, while FIG. 10 b illustrates twolayers of double layer crossover antenna 1101. Antenna 1101 may includea first coil of wire 1102 arranged on a first side of a dielectriccarrier 1104 and a second coil of wire 1103 on a second side of adielectric carrier 1104.

Arranging the antenna coils in a double layer may serve to increase thetransmission range of the antenna without increasing the size of theantenna. Such an arrangement, however, may also serve to increasecapacitance between the wires of each coil. In each wire coil, an amountof parasitic capacitance between wires may partially depend on thedistance each wire is from its neighbor. In a single layer coil,capacitance may be generated between each loop of the coil and itsneighbors to either side. Thus, more compact coils may generate moreparasitic capacitance. When a second layer coil is added, additionalcapacitance may then be generated between the wires of the first coiland the wires of the second coil. This additional capacitance may befurther increased if corresponding loops of the first and second coilshave the same or similar diameters, and/or if a dielectric carrierseparating the loops is made very thin. Increased parasitic capacitancein an antenna may serve to alter characteristics, such as resonantfrequency, of the antenna in unpredictable amounts based onmanufacturing specifications. Additionally, resonant frequency drift,caused, for example by moisture incursion or antenna flexing, may beincreased by the presence of increased parasitic capacitance. Thus, inorder to decrease variability in the manufactured product, it may beadvantageous to reduce the levels of parasitic capacitance in a duallayer antenna.

FIG. 10 b illustrates a double layer crossover antenna 1101 which mayserve to reduce the parasitic capacitance in a manufactured antenna. Asillustrated in FIG. 10 b, a first coil of wire 1102 is concentricallyoffset from a second coil of wire 1103. In contrast to a configurationwhere each loop of a first coil 1102 has the same diameter ascorresponding loop of the second coil 1103, concentrically offsettingcorresponding loops of each wire coil serves to increase the distancebetween a single loop of the first coil 1102 with a corresponding loopof the second coil 1103. This increased distance, in turn, may decreasethe parasitic wire-to-wire capacitance between loops of first coil 1102and corresponding loops of second coil 1103. This configuration may beparticularly advantageous in reducing parasitic capacitance in asituation where a dielectric carrier 1104 is thin enough such that theconcentric distance by which each coil is offset is relatively largecompared to the thickness of the dielectric carrier 1104. For example,in a situation where a dielectric carrier is 0.5 mm thick, a concentricoffset of 0.5 mm or more may produce a large change in parasiticcapacitance. In contrast, in a situation where a dielectric carrier is 5mm thick, a concentric offset of 0.5 mm may produce a smaller change inparasitic capacitance. The concentric offset between a first coil 1102and a second coil 1103 may be achieved, for example, by a plurality ofelectrical trace steps 1105 that offset each loop of the coils from eachpreceding loop. Electrical trace steps 1105 on a first side ofdielectric carrier 1104 cross over electrical trace steps 1105 on asecond side of dielectric carrier 1104, thus providing the crossoverfeature of double-layer crossover antenna 1101.

In additional embodiments, double layer crossover antenna 1101 mayinclude openings 1106 in dielectric carrier 1104 to facilitate theelectrical connection of first and second coils 1102, 1103. First andsecond coils 1102, 1103 of double layer crossover antenna 1101 may alsoinclude exposed electrical portions 1108 configured to electricallyconnect with a connector of a device housing that may be coupled toantenna 1101. Exposed electrical portions 1108 may be configured so asto maintain electrical contact with the connector of a device housingindependent of the axial orientation of the connection. As shown in FIG.10 a, for example, exposed electrical portions 1108 may be configured ascontinuous or discontinuous circles in order to achieve this. A firstexposed electrical portion 1108 configured as a discontinuous circle mayprovide a space through which an electrical trace may pass withoutcontacting the first exposed electrical portion, for example to connectwith a second exposed electrical portion located inside the first, or toother components located within the circle of the first exposedelectrical portion 1108. FIG. 10 a illustrates an antenna havingsubstantially elliptical coils; other shapes, such as circular,triangular, square, etc., may be also be used in different embodiments.Elliptical coils may facilitate placement of external unit 120 incertain areas (e.g., under the chin of a subject) while maintainingdesirable electrical performance characteristics.

FIGS. 11 a and 11 b illustrate an exemplary embodiment of external unit120, including features that may be found in any combination in otherembodiments. FIG. 11 a illustrates a side view of external unit 120,depicting carrier 1201 and electronics housing 1202.

Carrier 1201 may include a skin patch configured for adherence to theskin of a subject, for example through adhesives of mechanical means.Carrier 1201 may be flexible or rigid, or may have flexible portions andrigid portions. Carrier 1201 and may include a primary antenna 150, forexample, a double-layer crossover antenna 1101 such as that illustratedin FIGS. 10 a and 10 b. Carrier 1201 may also include power source 140,such as a paper battery, thin film battery, or other type ofsubstantially flat and/or flexible battery. Carrier 1201 may alsoinclude any other type of battery or power source. Carrier 1201 may alsoinclude a connector 1203 configured for selectively or removablyconnecting carrier 1201 to electronics housing 1202. Connector 1203 mayextend or protrude from carrier 1201. Connector 1203 may be configuredto be received by a recess 1204 of electronics housing 1202 Connector1203 may be configured as a non-pouch connector, configured to provide aselective connection to electronics housing 1204 without the substantialuse of concave feature. Connector 1203 may include, for example a peg,and may have flexible arms. Connector 1203 may further include amagnetic connection, a velcro connection, and/or a snap dome connection.Connector 1203 may also include a locating feature, configured to locateelectronics housing 1202 at a specific height, axial location, and/oraxial orientation with respect to carrier 1201. A locating feature ofconnector 1203 may further include pegs, rings, boxes, ellipses, bumps,etc. Connector 1203 may be centered on carrier 1201, may be offset fromthe center by a predetermined amount, or may be provided at any othersuitable location of carrier 1201. Multiple connectors 1203 may beprovided on carrier 1201. Connector 1203 may be configured such thatremoval from electronics housing 1202 causes breakage of connector 1203.Such a feature may be desirable to prevent re-use of carrier 1201, whichmay lose some efficacy through continued use.

Electronics housing 1202 is illustrated in side view in FIG. 11 a and ina bottom view in FIG. 11 b. Electronics housing 1202 may includeelectronics portion 1205, which may be arranged inside electronicshousing 1202 in any manner that is suitable. Electronics portion 1205may include various components, further discussed below, of externalunit 120. For example, electronics portion 1205 may include anycombination of at least one processor 144 associated with external unit120, a power source 140, such as a battery, a primary antenna 152, andan electrical circuit 170. Electronics portion 1205 may also include anyother component described herein as associated with external unit 120.Additional components may also be recognized by those of skill in theart.

Electronics housing 1202 may include a recess 1204 configured to receiveconnector 1203. Electronics housing 1202 may include at least oneelectrical connector 1210, 1211, 1212. Electrical connectors 1210, 1211,1212 may be arranged with pairs of electrical contacts, as shown in FIG.11 b, or with any other number of electrical contacts. The pair ofelectrical contacts of each electrical connector 1210, 1211, 1212 may becontinuously electrically connected with each other inside of housing1202, such that the pair of electrical contacts represents a singleconnection point to a circuit. In such a configuration, it is onlynecessary that one of the electrical contacts within a pair beconnected. Electrical connectors 1210, 1211, and 1212 may thus includeredundant electrical contacts. The electrical contacts of eachelectrical connector 1210, 1211, 1212 may also represent opposite endsof a circuit, for example, the positive and negative ends of a batterycharging circuit. In an exemplary embodiment, as shown in FIG. 11 b,electrical connectors 1210, 1211, and 1212 are configured so as tomaintain electrical contact with an exposed electrical portion 1108independent of an axial orientation of electronics housing 1202.Connection between any or all of electrical connectors 1210, 1211, 1212and exposed electrical portions 1108 may thus be established andmaintained irrespective of relative axial positions of carrier 1201 andhousing 1202. Thus, when connector 1203 is received by recess 1204,housing 1202 may rotate with respect to carrier 1201 withoutinterrupting electrical contact between at least one of electricalconnectors 1210, 1211, 1212 and exposed electrical portions 1108. Axialorientation independence may be achieved, for example, through the useof circular exposed electrical portions 1108 and each of a pair ofcontacts of electrical connectors 1210, 1211, 1212 disposed equidistantfrom a center of recess 1204 at a radius approximately equal to that ofa corresponding exposed electrical portion 1108. In this fashion, evenif exposed electrical portion 1108 includes a discontinuous circle, atleast one electrical contact of electrical connectors 1210, 1211, and1212 may make contact. In FIG. 11 b, electrical connectors 1210, 1211,1212 are illustrated as pairs of rectangular electrical contacts.Electrical connectors 1210, 1211, 1212, however, may include any numberof contacts, be configured as continuous or discontinuous circles, orhave any other suitable shape or configuration.

One exemplary embodiment may operate as follows. As shown in FIG. 11 b,electronics housing 1202 may include more electrical connectors 1210,1211, 1212, than a carrier 1201 includes exposed electrical portions1108. In the illustrated embodiments, electronics housing 1202 includesthree electrical connectors 1210, 1211, and 1212, while a double-layercrossover antenna 1101 includes two exposed electrical portions 1108. Insuch an embodiment, two electrical connectors 1211 and 1212 may beconfigured with continuously electrically connected electrical contacts,such that each connector makes contact with a different exposedelectrical portion 1108, where the exposed electrical portions 1108represent opposite ends of double layer crossover antenna 1101. Thus,antenna 1101 may be electrically connected to the electrical componentscontained in electronics portion 1205. When connected to carrier 1201 inthis configuration, electrical connectors 1210 may not make contact withany electrodes. In this embodiment, electrical connectors 1210 may bereserved to function as opposite ends of a battery charging circuit, inorder to charge a battery contained in electronics portion 1205 whenelectronics housing 1202 is not being used for therapy. A batterycharger unit may be provided with a non-breakable connector similar tothat of non-pouch connector 1203, and configured to engage with recess1204. Upon engaging with recess 1204, electrode contacts of the batterycharger unit may contact electrical connectors 1210 to charge a batterycontained within electronics portion 1205.

In an additional embodiment consistent with the present disclosure, anactivator chip may include electronics housing 1202. Processor 144 maybe configured to activate when at least one of electrical connectors1210, 1211, 1212 contact exposed electrical portions 1108 included incarrier 1201. In this manner, an electronics housing 1202 may be chargedand left dormant for many days prior to activation. Simply connectingelectronics housing 1202 to carrier 1201 (and inducing contact betweenan electrical connector 1210, 1211, 1212 and an electrode portion 1108)may cause the processor to activate. Upon activation, processor 144 maybe configured to enter a specific mode of operation, such as acalibration mode (for calibrating the processor after placement of thecarrier on the skin), a placement mode (for assisting a user to properlyplace the carrier on the skin), and/or a therapy mode (to begin atherapy session). The various modes of processor 144 may include waitingperiods at the beginning, end, or at any time during. For example, aplacement mode may include a waiting period at the end of the mode toprovide a period during which a subject may fall asleep. A therapy modemay include a similar waiting period at the beginning of the mode.Additionally or alternatively, processor 144 may be configured toprovide waiting periods separate from the described modes, in order toprovide a desired temporal spacing between system activities.

Implant unit 110 may additionally include a plurality offield-generating implant electrodes 158 a, 158 b. The electrodes mayinclude any suitable shape and/or orientation on the implant unit solong as the electrodes may be configured to generate an electric fieldin the body of a patient. Implant electrodes 158 a and 158 b may alsoinclude any suitable conductive material (e.g., copper, silver, gold,platinum, iridium, platinum-iridium, platinum-gold, conductive polymers,etc.) or combinations of conductive (and/or noble metals) materials. Insome embodiments, for example, the electrodes may include short lineelectrodes, circular electrodes, and/or circular pairs of electrodes. Asshown in FIG. 4, electrodes 158 a and 158 b may be located on an end ofa first extension 162 a of an elongate arm 162. The electrodes, however,may be located on any portion of implant unit 110. Additionally, implantunit 110 may include electrodes located at a plurality of locations, forexample on an end of both a first extension 162 a and a second extension162 b of elongate arm 162, as illustrated, for example, in FIG. 5.Implant electrodes may have a thickness between about 200 nanometers and1 millimeter. Anode and cathode electrode pairs may be spaced apart byabout a distance of about 0.2 mm to 25 mm. In additional embodiments,anode and cathode electrode pairs may be spaced apart by a distance ofabout 1 mm to 10 mm, or between 4 mm and 7 mm. Adjacent anodes oradjacent cathodes may be spaced apart by distances as small as 0.001 mmor less, or as great as 25 mm or more. In some embodiments, adjacentanodes or adjacent cathodes may be spaced apart by a distance betweenabout 0.2 mm and 1 mm.

FIG. 4 provides a schematic representation of an exemplary configurationof implant unit 110. As illustrated in FIG. 4, in one embodiment, thefield-generating electrodes 158 a and 158 b may include two sets of fourcircular electrodes, provided on flexible carrier 161, with one set ofelectrodes providing an anode and the other set of electrodes providinga cathode. Implant unit 110 may include one or more structural elementsto facilitate implantation of implant unit 110 into the body of apatient. Such elements may include, for example, elongated arms, sutureholes, polymeric surgical mesh, biological glue, spikes of flexiblecarrier protruding to anchor to the tissue, spikes of additionalbiocompatible material for the same purpose, etc. that facilitatealignment of implant unit 110 in a desired orientation within apatient's body and provide attachment points for securing implant unit110 within a body. For example, in some embodiments, implant unit 110may include an elongate arm 162 having a first extension 162 a and,optionally, a second extension 162 b. Extensions 162 a and 162 b may aidin orienting implant unit 110 with respect to a particular muscle (e.g.,the genioglossus muscle), a nerve within a patient's body, or a surfacewithin a body above a nerve. For example, first and second extensions162 a, 162 b may be configured to enable the implant unit to conform atleast partially around soft or hard tissue (e.g., nerve, bone, ormuscle, etc.) beneath a patient's skin. Further, implant unit 110 mayalso include one or more suture holes 160 located anywhere on flexiblecarrier 161. For example, in some embodiments, suture holes 160 may beplaced on second extension 162 b of elongate arm 162 and/or on firstextension 162 a of elongate arm 162. Implant unit 110 may be constructedin various shapes. Additionally, or alternatively, implant unit 110 mayinclude surgical mesh 1050 or other perforatable material. In someembodiments, implant unit may appear substantially as illustrated inFIG. 4. In other embodiments, implant unit 110 may lack illustratedstructures such as second extension 162 b, or may have additional ordifferent structures in different orientations. Additionally, implantunit 110 may be formed with a generally triangular, circular, orrectangular shape, as an alternative to the winged shape shown in FIG.4. In some embodiments, the shape of implant unit 110 (e.g., as shown inFIG. 4) may facilitate orientation of implant unit 110 with respect to aparticular nerve to be modulated. Thus, other regular or irregularshapes may be adopted in order to facilitate implantation in differingparts of the body.

As illustrated in FIG. 4, secondary antenna 152 and electrodes 158 a,158 b may be mounted on or integrated with flexible carrier 161. Variouscircuit components and connecting wires (discussed further below) may beused to connect secondary antenna with implant electrodes 158 a and 158b. To protect the antenna, electrodes, circuit components, andconnecting wires from the environment within a patient's body, implantunit 110 may include a protective coating that encapsulates implant unit110. In some embodiments, the protective coating may be made from aflexible material to enable bending along with flexible carrier 161. Theencapsulation material of the protective coating may also resisthumidity penetration and protect against corrosion. In some embodiments,the protective coating may include a plurality of layers, includingdifferent materials or combinations of materials in different layers

FIG. 5 is a perspective view of an alternate embodiment of an implantunit 110, according to an exemplary embodiment of the presentdisclosure. As illustrated in FIG. 5, implant unit 110 may include aplurality of electrodes, located, for example, at the ends of firstextension 162 a and second extension 162 b. FIG. 5 illustrates anembodiment wherein implant electrodes 158 a and 158 b include short lineelectrodes.

Returning to FIGS. 2 and 3, external unit 120 may be configured tocommunicate with implant unit 110. For example, in some embodiments, aprimary signal may be generated on primary antenna 150, using, e.g.,processor 144, signal source 142, and amplifier 146. More specifically,in one embodiment, power source 140 may be configured to provide powerto one or both of the processor 144 and the signal source 142. Theprocessor 144 may be configured to cause signal source 142 to generate asignal (e.g., an RF energy signal). Signal source 142 may be configuredto output the generated signal to amplifier 146, which may amplify thesignal generated by signal source 142. The amount of amplification and,therefore, the amplitude of the signal may be controlled, for example,by processor 144. The amount of gain or amplification that processor 144causes amplifier 146 to apply to the signal may depend on a variety offactors, including, but not limited to, the shape, size, and/orconfiguration of primary antenna 150, the size of the patient, thelocation of implant unit 110 in the patient, the shape, size, and/orconfiguration of secondary antenna 152, a degree of coupling betweenprimary antenna 150 and secondary antenna 152 (discussed further below),a desired magnitude of electric field to be generated by implantelectrodes 158 a, 158 b, etc. Amplifier 146 may output the amplifiedsignal to primary antenna 150.

External unit 120 may communicate a primary signal on primary antenna tothe secondary antenna 152 of implant unit 110. This communication mayresult from coupling between primary antenna 150 and secondary antenna152. Such coupling of the primary antenna and the secondary antenna mayinclude any interaction between the primary antenna and the secondaryantenna that causes a signal on the secondary antenna in response to asignal applied to the primary antenna. In some embodiments, couplingbetween the primary and secondary antennas may include capacitivecoupling, inductive coupling, radiofrequency coupling, etc. and anycombinations thereof.

Coupling between primary antenna 150 and secondary antenna 152 maydepend on the proximity of the primary antenna relative to the secondaryantenna. That is, in some embodiments, an efficiency or degree ofcoupling between primary antenna 150 and secondary antenna 152 maydepend on the proximity of the primary antenna to the secondary antenna.The proximity of the primary and secondary antennas may be expressed interms of a coaxial offset (e.g., a distance between the primary andsecondary antennas when central axes of the primary and secondaryantennas are co-aligned), a lateral offset (e.g., a distance between acentral axis of the primary antenna and a central axis of the secondaryantenna), and/or an angular offset (e.g., an angular difference betweenthe central axes of the primary and secondary antennas). In someembodiments, a theoretical maximum efficiency of coupling may existbetween primary antenna 150 and secondary antenna 152 when both thecoaxial offset, the lateral offset, and the angular offset are zero.Increasing any of the coaxial offset, the lateral offset, and theangular offset may have the effect of reducing the efficiency or degreeof coupling between primary antenna 150 and secondary antenna 152.

As a result of coupling between primary antenna 150 and secondaryantenna 152, a secondary signal may arise on secondary antenna 152 whenthe primary signal is present on the primary antenna 150. Such couplingmay include inductive/magnetic coupling, RF coupling/transmission,capacitive coupling, or any other mechanism where a secondary signal maybe generated on secondary antenna 152 in response to a primary signalgenerated on primary antenna 150. Coupling may refer to any interactionbetween the primary and secondary antennas. In addition to the couplingbetween primary antenna 150 and secondary antenna 152, circuitcomponents associated with implant unit 110 may also affect thesecondary signal on secondary antenna 152. Thus, the secondary signal onsecondary antenna 152 may refer to any and all signals and signalcomponents present on secondary antenna 152 regardless of the source.

While the presence of a primary signal on primary antenna 150 may causeor induce a secondary signal on secondary antenna 152, the couplingbetween the two antennas may also lead to a coupled signal or signalcomponents on the primary antenna 150 as a result of the secondarysignal present on secondary antenna 152. A signal on primary antenna 150induced by a secondary signal on secondary antenna 152 may be referredto as a primary coupled signal component. The primary signal may referto any and all signals or signal components present on primary antenna150, regardless of source, and the primary coupled signal component mayrefer to any signal or signal component arising on the primary antennaas a result of coupling with signals present on secondary antenna 152.Thus, in some embodiments, the primary coupled signal component maycontribute to the primary signal on primary antenna 150.

Implant unit 110 may be configured to respond to external unit 120. Forexample, in some embodiments, a primary signal generated on primary coil150 may cause a secondary signal on secondary antenna 152, which inturn, may cause one or more responses by implant unit 110. In someembodiments, the response of implant unit 110 may include the generationof an electric field between implant electrodes 158 a and 158 b.

FIG. 6 illustrates circuitry 170 that may be included in external unit120 and circuitry 180 that may be included in implant unit 110.Additional, different, or fewer circuit components may be included ineither or both of circuitry 170 and circuitry 180. As shown in FIG. 6,secondary antenna 152 may be arranged in electrical communication withimplant electrodes 158 a, 158 b. In some embodiments, circuitryconnecting secondary antenna 152 with implant electrodes 158 a and 158 bmay cause a voltage potential across implant electrodes 158 a and 158 bin the presence of a secondary signal on secondary antenna 152. Thisvoltage potential may be referred to as a field inducing signal, as thisvoltage potential may generate an electric field between implantelectrodes 158 a and 158 b. More broadly, the field inducing signal mayinclude any signal (e.g., voltage potential) applied to electrodesassociated with the implant unit that may result in an electric fieldbeing generated between the electrodes.

Energy transfer between primary antenna 150 and secondary antenna 152via the primary signal may be improved when a resonant frequency ofprimary antenna 150 and its associated circuitry 170 matches that ofsecondary antenna 152 and its associated circuitry 180. As used herein aresonant frequency match between two antennas may be characterized bythe proximity of two resonant frequencies to one another. For example, aresonant frequency match may be considered to occur when two resonantfrequencies are within 30%, 20%, 10%, 5%, 3%, 1%, 0.5%, 0.1%, or less ofeach other. Accordingly, a resonant frequency mismatch may be consideredto occur when two resonant frequencies do not match. The proximity ofthe two resonant frequencies required to be considered a match maydepend on the circumstances of energy transfer between the two antennas.A resonant frequency match between two antennas may also becharacterized by the efficiency of energy transfer between the antennas.The efficiency of energy transfer between two antennas may depend onseveral factors, one of which may be the degree to which the resonantfrequencies of the antennas match. Thus, if all other factors are heldconstant, changing the resonant frequency of one antenna with respect tothe other will alter the efficiency of energy transfer. A resonantfrequency match between two antennas may be considered to occur when theefficiency of energy transfer is within 50% or greater of a maximumenergy transfer when all other factors remain constant. In someembodiments, a resonant frequency match may require energy transferefficiencies of 60%, 70%, 80%, 90%, 95% or greater.

Several embodiments are provided in order to appropriately matchresonant frequencies between a primary signal and a secondary antenna152. Because the secondary antenna 152 is intended for implantation withimplant unit 110, it may be difficult to adjust the resonant frequencyof the antenna during use. Furthermore, due to the possibility ofmoisture incursion into a primary capsule encapsulating implant unit110, implant circuitry 180, and secondary antenna 152, a resonantfrequency of the implant unit 110 may drift after implantation. Otherfactors present during implantation may also influence the frequencydrift of implant unit 110 after implantation. This drifting of theresonant frequency may last for several days to several months afterimplantation before stabilizing. For example, the resonant frequency ofan implant unit 110 may drift from 8.1 kHz to 7.9 kHz. Throughexperimentation or simulation, it may be possible to predict by how muchthe resonant frequency may drift. Thus, using the example above, if along term resonant frequency value of 7.9 kHz is desired, an implantunit 110 may be manufactured with a resonant frequency value of 8.1 kHzprior to implantation.

Resonant frequency values of manufactured implant units 110 may beadjusted during the manufacturing process through the use of at leastone trimming capacitor. In one embodiment, a carrier 161 may bemanufactured with all or some of the components of the final implantunit, including, for example, secondary antenna 152, implant circuitry180, modulation electrodes 158 a, 158 b. The resonant frequency of thisassembly may then be measured or otherwise determined. Due to variationsin manufacturing processes and materials, the resonant frequency of eachmanufactured unit may differ. Thus, in order to meet a specificresonance frequency, each implant unit may be adjusted through theaddition of one or more trimming capacitors to the implant circuitry 180prior to encapsulation. In one embodiment, a capacitor may be lasertrimmed to a predetermined capacitance value before insertion intoimplant circuitry 180. In another embodiment, a stock capacitor of knownvalue may be inserted into implant circuitry 180. In still anotherembodiment, a plurality of capacitors may be inserted into implantedcircuitry 180 to appropriately adjust the resonant frequency of implantunit 110. Such a plurality of capacitors may include a series ofcapacitors having progressively smaller capacitance values, and aresonant frequency of the assembly may be measured after the insertionof each capacitor prior to choosing and inserting the next. In thisfashion, implantable circuit 180 may include at least one capacitorconfigured to create a predetermined mismatch between a resonantfrequency of implantable circuit 180 and external circuit 170.

In addition to resonant frequency drift in implant unit 110, a resonantfrequency of primary antenna 152 may vary temporally due, for example,to changing environmental conditions. For example, the application ofthe antenna 152 to the skin of a subject may cause bending of theprimary antenna 152 to conform to the skin of a subject. Such bendingmay cause a shift in the spatial relationship among coils within primaryantenna 152, which may lead to a change in resonant frequency. Otherfactors associated, e.g., with the condition of the subject's skin(e.g., moisture, sweat, oil, hair, etc.) may also change over time andmay contribute to a change in resonant frequency of the primary antenna152. The temporal variance in resonant frequency of primary antenna maybe between 0-5%, up to 10%, or up to 20% or more. Thus, a currentresonant frequency of the primary antenna 152 (e.g., a resonantfrequency exhibited by the primary antenna or a circuit including theprimary antenna at a particular point in time or over a time duration)may be adjusted to counter effects of changing environmental conditionsas well as to match frequencies with a resonant frequency of animplantable circuit 180.

Processor 144 of the external unit may be configured to determine aresonant frequency mismatch between an external circuit 170 associatedwith primary antenna 152 and an implantable circuit 180 associated withsecondary antenna 150 and adjust a resonant frequency of the externalcircuit 170 associated with primary antenna 152 in order to reduce oreliminate the resonant frequency mismatch. During transmission of aprimary signal from a primary antenna to a secondary antenna, processor144 may be configured to determine a resonant frequency mismatch basedon a primary coupled signal component present on the primary antenna duecoupling between primary antenna 152 and secondary antenna 150.Monitoring a primary coupled signal component by the processor 144 mayprovide an indication of transmission efficiency, which may in turn bean indication of resonant frequency mismatch. The primary coupled signalcomponent and the interaction between primary antenna 152 and secondaryantenna 150 are explained in greater detail below.

Upon determining a resonant frequency mismatch between an externalcircuit 170 associated with primary antenna 152 and an implantablecircuit 180 associated with secondary antenna 150, processor 144 mayadjust the resonant frequency of the external circuit 170 to reduce themismatch. A resonance matching unit 190 may be in electricalcommunication with the external circuit 170 to facilitate the alterationof the resonant frequency of the external circuit 170. The circuitformed between the external circuit 170 and the resonance matching unit190 may include an adjustable resonance circuit 191. Such alteration maybe performed, for example, through the selective inclusion or exclusionof at least one capacitor into or out of the adjustable resonancecircuit 191 formed between the external circuit 170 and the resonancematching unit 190. Adding (or subtracting) capacitors in resonancematching unit 190 may cause a change in the resonant frequency of theadjustable resonance circuit 191. Resonance matching unit 190 may beprovided with one or more trim capacitors configured, through processor144 controlled switches, for selective inclusion and exclusion. Theswitches may include, for example, transistors or relays. Thus,processor 144 may include or exclude a capacitor adjustable resonancecircuit 191 by opening or closing a switch associated with therespective capacitor. Providing a single capacitor, therefore, permitsprocessor 144 to switch the resonant frequency of the adjustableresonance circuit 191 between two different values. In an exemplaryembodiment, a bank of six capacitors may be provided, permittingprocessor 144 to switch the resonant frequency of the adjustableresonance circuit 191 between 64 (i.e., 2⁶) different values. Inalternative embodiments, more or fewer capacitors may be provided foradjusting the resonant frequency of the adjustable resonance circuit 191to reduce the mismatch.

In an exemplary embodiment, processor 144 may be configured to switchcapacitors from a capacitor bank into and out of the adjustableresonance circuit 191 during transmission of a primary signal todetermine a capacitor combination that changes (e.g., increases)transmission efficiency and resonant frequency match. In someembodiments, processor 144 may be configured to select an optimalcombination of capacitors to provide a best resonant frequency match. Insome embodiments, processor 144 may be configured to select an optimalcombination of capacitors, based on a detected magnitude of the powertransmitted to the implant unit. In alternative embodiments, processor144 may be configured to select a combination of capacitors thatprovides a resonant frequency match surpassing a predeterminedthreshold, regardless of whether such combination produces an optimalresonant frequency match.

Resonant frequency matching between a primary antenna 150 and asecondary antenna 152 may increase the efficiency of energy transferbetween the antennas. In additional embodiments, external unit 120 maybe provided with features that may facilitate development of a matchbetween a frequency of a primary signal and a resonant frequency of theexternal circuit 170 associated with primary antenna 150. Development ofsuch a match may further increase the efficiency of external unit 120.Some conventional amplifiers may be configured to generate a signal at asingle frequency. Using such an amplifier may create a mismatch betweenthe signal generated by the amplifier and the resonant frequency of theadjustable resonance circuit 191 when the adjustable resonance circuit191 is adjusted to match frequencies with an implant unit 110. Thus, insome embodiments, self-resonant circuits may be provided for use with aresonance matching unit 190, as further described below with respect toFIGS. 7 and 8.

FIG. 7 depicts an embodiment illustrating a self-resonant transmittercircuit employing a modified class D amplifier for use with resonantfrequency matching methods. Modified class D amplifier 1600 may be usedin place of, or in addition to, any or all of the elements of externalunit 120 depicted in FIG. 3. For example, modified class D amplifier1600 may replace signal source 142 and amplifier 146. In thisembodiment, processor 144 may be configured to adjust the operation of aclass D amplifier to provide a frequency match between a generatedsignal and a resonant frequency of an adjustable resonance circuit 191.Because the resonant frequency adjustable resonance circuit 191 may beadjusted to match that of implanted circuit 180 associated withsecondary antenna 152 during operation, it may be beneficial to adjustthe frequency of the generated signal as well to improve efficiencywithin the adjustable resonance circuit 191 external unit 120. Modifiedclass D amplifier 1600 may be used to provide such an adjustment asfollows. Modified class D amplifier 1600 includes an H bridge 1601including switches (such as MOSFETs) 1620. Between the switches, anadjustable resonance transmitter circuit 1610 may be provided. Power tothe modified class D amplifier may be provided by supply 1650, which mayinclude a battery, for example. Adjustable resonance transmitter circuit1610 may include adjustable resonance circuit 191. Thus, adjustableresonance transmitter circuit 1610 may include at least a primaryantenna 150 and a resonance matching unit 190. FIG. 7 illustrates anexemplary self-resonant transmitter circuit 1610, depicting multiplecapacitances 1640 and inductances 1660. In some embodiments, resonancematching unit 190 may take the place of some or all of capicitances1640. In some embodiments, adjustable resonance transmitter circuit 1610may include a primary antenna 150, and thus inductance 1660 may includethe inductance of antenna 150. Capacitances 1640 may include multiplecapacitors, combinations of which may be chosen from among trimcapacitors as described above, in order to selectively provide anappropriate value of capacitance 1640. The value of capacitance 1640 maybe selected for resonant frequency matching to adjustable resonancecircuit 191. Inductances 1660 may be provided at least partially byprimary antenna 150. Processor 144 may also adjust a driving frequencyof the H bridge switches 1620 in order to generate a signal of afrequency that matches the resonant frequency of self-resonant circuit1610. By selectively opening and closing switches 1620 appropriately,the DC voltage signal of supply 1650 may be converted into a square waveof a selected frequency. This frequency may be selected to match theresonant frequency of adjustable resonance circuit 1610 in order tochange (e.g., increase) the efficiency of the circuit.

FIG. 8 depicts an additional embodiment illustrating a pulsed modeself-resonant transmitter 1700 for use with resonant frequency matchingmethods. Pulsed mode self-resonant transmitter 1700 may be used in placeof, or in addition to, any or all of the elements of external unit 120depicted in FIG. 3. For example, pulsed mode self-resonant transmitter1700 may replace signal source 142 and amplifier 146. In thisembodiment, processor 144 may be configured to control the circuitthrough a power release unit, depicted in the present embodiment asswitch 1730. A power release unit may include a transistor, relay, orsimilar switching device. Pulsed mode self-resonant transmitter 1700 mayinclude a primary power source 1780, for example, a battery oralternative source of power. Transmitter 1700 may include a powerstorage unit, such as storage capacitor 1750. Other suitable powerstorage units may also be utilized, such as an inductor and/or battery,as well as combinations of these storage elements. Transmitter 1700 mayalso include a self-resonant transmitter circuit 1710, includingresonance capacitance 1720 and a resonance inductance 1760. Resonanceinductance 1760 may be provided at least partially by primary antenna150.

Transmitter 1700 may operate in the following manner, among others.Processor 144 may be configured to control the power release unit,illustrated in FIG. 8 as switch 1730. Processor 144 may be configured tocontrol a mode of operation of the power release unit such that in afirst mode of operation the power storage unit receives power from thepower source, and in a second mode of operation, the power storage unitreleases power to the antenna to cause an oscillation in the primaryantenna at a resonance frequency.

For example, in a first mode of operation, the power storage unit, forexample, storage capacitor 1750, may be configured to store energy frompower source 1780. When switch 1730 is maintained in an open position,or a first state, current from power source 1780 may flow into storagecapacitor 1750 which thereby stores energy by accumulating electricalcharge.

The power release unit may be configured to cause the release a pulse ofenergy from the power storage unit to primary antenna 150 after thepower storage unit has accumulated certain amount electrical charge. Ina second mode of operation, when switch 1730 is closed, or moved to asecond state, charged storage in capacitor 1750 drives current into theself-resonant circuit 1710 during a current loading period, where energyis stored in inductance 1760. Due to the operation of diode 1770,current flow into circuit 1710 will be cut off after a period of energyaccumulation. The current transferred to circuit 1710 will thenoscillate freely within circuit 1710 at the resonant frequency ofcircuit 1710 and thus generate a signal for transmission to the implantthrough primary antenna 150 (which is included in the circuit andcreates at least a portion of inductance 1760). During this freeoscillation period, the amplitude of the oscillations may diminish at arate determined by the components of circuit 1710. When the oscillationshave diminished to a desired level, for example, to between 5% and 10%,between 10% and 50%, or between 50% and 90% of an initial amplitude,switch 1730 may be closed again to permit storage capacitor 1750 toenter a current loading period again. During the free oscillationperiod, oscillation amplitude may be measured, for example, by processor144 to determine the appropriate operation of switch 1730. In someembodiments, a current loading period may be between 0.5 and 10 μs, orbetween 2 and 5 μs. In some embodiments, a free oscillation period maybe between 10 and 30 μs. Switch 1730 may be opened and closed severaltimes in order to maintain a desired level of current flow in circuit1710 for a desired period of time by assembling a series of currentloading period and free oscillation periods. The desired period of timemay correspond to a sub-pulse of a stimulation control signal, discussedin greater detail below. The operation of switch 1730 may be adjusted byprocessor 144 to increase the efficacy of modulation signals generatedin implant unit 110. Such adjustments may be based, for example, onfeedback received from implant unit 110, as discussed herein.

Storage capacitor 1750 may be selected to store enough energy for acomplete stimulation sub-pulse, which may be, for example, between 50and 250 μs, or between 1 μs and 2 ms. After delivery of a stimulationsub-pulse, switch 1730 may be closed again in order to permit storagecapacitor 1750 to accumulate energy from power source 1780. Suchaccumulation may occur, for example, over a time period of between 5 and50 milliseconds. Energy accumulated over this accumulation period maybe, for example, between 1 microjoule and 10 millijoules.

Storage capacitor 1750 may be selected so as to have characteristicspermitting the current in circuit 1710 to rise to a desired level at adesired rate. In some embodiments, the capacitor 1750 may be selectedsuch that the current in circuit 1710 may change rapidly to permit acorresponding rapid increase in transmission power. For example,capacitor 1750 may be selected so as to permit the current in circuit1710 to rise to a desired value, (e.g. between 100 mAmps and 1 Amp)during the current loading period. In some embodiments, capacitor 1750may be selected such that a maximum value of current in circuit 1710 ishigher than may be possible using power source 1780 alone. For example,capacitor 1750 may be selected so as to drive a current in circuit 1710that is 5, 10, 20, or 100 times greater than may be permitted throughthe use of power source 1780 alone. For example, by utilizing the pulsedmode resonant transmitter, a battery having a voltage between 3 and 6volts used as power source 1780 may yield a voltage in circuit 1710 ofbetween 300 and 600 volts. Suitable capacitors may be chosen in therange of 1 microfarad to 100 microfarads.

Circuit 1710 may be configured so as to permit a power storage unit torelease a desired portion of its stored energy in a desired timeperiod—i.e. the current loading period as described above. For example,circuit 1710 may be configured to permit a power storage unit, such asstorage capacitor 1750 to release between 1% and 10% of its accumulatedcharge in a time period between 1 and 10 microseconds.

Because the signal for transmission is generated by the self resonanceof circuit 1710, it has a frequency dictated by the resonant frequencyof circuit 1710. Such an arrangement may offer more efficienttransmission than configurations that drive a transmission circuit witha signal that does not match the resonant frequency of the transmissioncircuit. Generating a transmission signal in the manner described mayminimize the risk of reductions in energy transfer efficiency caused bychanges in resonance of the transmission circuit. As discussed above,such temporal resonance changes in the transmission circuit may rangebetween 0-5%, up to 10%, up to 20% or more.

Components of transmitter 1700 may be chosen such that the currentloading period is approximately two microseconds, and a period of freeoscillation in circuit 1710 is between 10 and 20 microseconds. Othercomponents may be selected, however, to provide any desired currentloading period or free oscillation period. As described elsewhere inthis disclosure, stimulation pulses of varying lengths may be desired.Stimulation pulses of longer than a single period of free oscillationmay be constructed by multiple cycles of loading and releasing energyfrom storage capacitor 1750 into circuit 1710. Storage capacitor 1750may itself be chosen to store enough charge to drive a large number ofoscillation cycles (e.g. between 10 and 100) in order to constructentire stimulation pulses without requiring recharging from power source1780.

In some embodiments, transmitter 1700 may be used in conjunction withresonance matching unit 190. As discussed above, due to changes inenvironmental conditions, a resonant frequency of circuit 1710 may vary.Processor 144 may be configured to cause delivery of an impulse ofenergy to circuit 1710, for example, via operation of a power releaseunit such as switch 1730. This impulse of energy may excite circuit 1710and cause oscillations in circuit 1710 at the current resonant frequencyof the external circuit such that power is delivered from primaryantenna 150 to implant unit 110 at the current resonant frequency of thecircuit 1710. Resonance matching unit 190 may be in electricalcommunication (e.g., direct or indirect electrical connection) withcircuit 1710, and may be configured, under control of processor 144, toalter the current resonant frequency of the circuit formed by circuit1710 and matching unit 190. Such alterations of the current resonantfrequency may be performed, for example, to optimize transmissionefficiency with implant unit 121.

Pulsed mode self-resonant transmitter 1700 may provide severaladvantages. As described above, because the transmission signal isgenerated by the self-resonance of circuit 1710, it likely will matchthe resonant frequency of circuit 1710, obviating a need to match thefrequency of the generated signal with the circuit resonance frequency.Further, because energy is stored in capacitor 1750 prior to dischargeinto circuit 1710, a greater flexibility in choice of power source 1780may be provided. Effective neural modulation by an implant unit maydepend on current levels that rise rapidly.

Naturally functioning neurons function by transmitting action potentialsalong their length. Structurally, neurons include multiple ion channelsalong their length that serve to maintain a voltage potential gradientacross a plasma membrane between the interior and exterior of theneuron. Ion channels operate by maintaining an appropriate balancebetween positively charged sodium ions on one side of the plasmamembrane and negatively charged potassium ions on the other side of theplasma membrane. A sufficiently high voltage potential differencecreated near an ion channel may exceed a membrane threshold potential ofthe ion channel. The ion channel may then be induced to activate,pumping the sodium and potassium ions across the plasma membrane toswitch places in the vicinity of the activated ion channel. This, inturn, further alters the potential difference in the vicinity of the ionchannel, which may serve to activate a neighboring ion channel. Thecascading activation of adjacent ion channels may serve to propagate anaction potential along the length of the neuron. Further, the activationof an ion channel in an individual neuron may induce the activation ofion channels in neighboring neurons that, bundled together, form nervetissue. The activation of a single ion channel in a single neuron,however, may not be sufficient to induce the cascading activation ofneighboring ion channels necessary to permit the propagation of anaction potential. Thus, the more ion channels in a locality that may berecruited by an initial potential difference, caused through naturalmeans such as the action of nerve endings or through artificial means,such as the application of electric fields, the more likely thepropagation of an action potential may be. The process of artificiallyinducing the propagation of action potentials along the length of anerve may be referred to as stimulation, or up modulation.

Neurons may also be prevented from functioning naturally throughconstant or substantially constant application of a voltage potentialdifference. After activation, each ion channel experiences a refractoryperiod, during which it “resets” the sodium and potassium concentrationsacross the plasma membrane back to an initial state. Resetting thesodium and potassium concentrations causes the membrane thresholdpotential to return to an initial state. Until the ion channel restoresan appropriate concentration of sodium and potassium across the plasmamembrane, the membrane threshold potential will remain elevated, thusrequiring a higher voltage potential to cause activation of the ionchannel. If the membrane threshold potential is maintained at a highenough level, action potentials propagated by neighboring ion channelsmay not create a large enough voltage potential difference to surpassthe membrane threshold potential and activate the ion channel. Thus, bymaintaining a sufficient voltage potential difference in the vicinity ofa particular ion channel, that ion channel may serve to block furthersignal transmission.

The membrane threshold potential may also be raised without eliciting aninitial activation of the ion channel. If an ion channel (or a pluralityof ion channels) are subjected to an elevated voltage potentialdifference that is not high enough to surpass the membrane thresholdpotential, it may serve to raise the membrane threshold potential overtime, thus having a similar effect to an ion channel that has not beenpermitted to properly restore ion concentrations. Thus, an ion channelmay be recruited as a block without actually causing an initial actionpotential to propagate. This method may be valuable, for example, inpain management, where the propagation of pain signals is undesired. Asdescribed above with respect to stimulation, the larger the number ofion channels in a locality that may be recruited to serve as blocks, themore likely the chance that an action potential propagating along thelength of the nerve will be blocked by the recruited ion channels,rather than traveling through neighboring, unblocked channels.

Slowly rising current levels in a modulation signal may affect a nervein a similar fashion. That is, a current level (and thus voltagepotential difference) that increases slowly may serve to raise themembrane threshold potential during the application of the modulationsignal, thus causing the nerve to require a higher ultimate currentlevel to achieve modulation, which may result in diminished or absentmodulation. Applying a modulation signal such that the current level (orpower level) of the signal increases rapidly or greater than a thresholdrate may not give the nerve time to react, and thus may ultimatelyproduce more effective modulation at lower current levels. In contrast,using a slow current ramp or a rate lower than a threshold rate mayultimately require higher levels of current to achieve effectiveneuromodulation. Thus, in order to operate in certain modes, the powertransferred to implant unit 110 may be required to increase at a ratethat surpasses a power increase threshold rate. To achieve this with abattery alone may require a high-voltage and/or high-current battery.Such a battery may be expensive and/or difficult to design. This needmay be obviated by transmitter 1700, which permits the delivery of arapidly changing current levels and high peak current levels through theuse of a power source incapable of delivering the threshold rate ofpower increase, such as a relatively low voltage/low current battery.

In some embodiments, a threshold rate of power increase in the implantmay include an increase in current flow through the implant fromsubstantially zero current to a current level sufficient for effectivemodulation (e.g., a current between 200 μAmps and 3 mAmps) in less than20 microseconds, less than 10 microseconds, or less than 5 microseconds.Thus, for example, a threshold rate of power increase may include acurrent increase between 0.01-0.6 milliamps per microsecond. In someembodiments, a threshold rate of power increase may include a currentincrease between 0.05-0.2 milliamps per microsecond.

Transmitter 1700 may also offer enhanced efficiency. Transmitter 1700also may use fewer switches (e.g. transistors) than does a conventionalamplifying circuit. Each switch may be a source of energy loss,contributing to an overall less efficient circuit. The presence of asingle switch 1730 in transmitter 1700 may increase the efficiency ofthe circuit as a whole.

The field inducing signal may be generated as a result of conditioningof the secondary signal by circuitry 180. As shown in FIG. 6, circuitry170 of external unit 120 may be configured to generate an AC primarysignal on primary antenna 150 that may cause an AC secondary signal onsecondary antenna 152. In certain embodiments, however, it may beadvantageous (e.g., in order to generate a unidirectional electric fieldfor modulation of a nerve) to provide a DC field inducing signal atimplant electrodes 158 a and 158 b. To convert the AC secondary signalon secondary antenna 152 to a DC field inducing signal, circuitry 180 inimplant unit 110 may include an AC-DC converter. The AC to DC convertermay include any suitable converter known to those skilled in the art.For example, in some embodiments the AC-DC converter may includerectification circuit components including, for example, diode 156 andappropriate capacitors and resistors. In alternative embodiments,implant unit 110 may include an AC-AC converter, or no converter, inorder to provide an AC field inducing signal at implant electrodes 158 aand 158 b.

As noted above, the field inducing signal may be configured to generatean electric field between implant electrodes 158 a and 158 b. In someinstances, the magnitude and/or duration of the generated electric fieldresulting from the field inducing signal may be sufficient to modulateone or more nerves in the vicinity of electrodes 158 a and 158 b. Insuch cases, the field inducing signal may be referred to as a modulationsignal. In other instances, the magnitude and/or duration of the fieldinducing signal may generate an electric field that does not result innerve modulation. In such cases, the field inducing signal may bereferred to as a sub-modulation signal.

Various types of field inducing signals may constitute modulationsignals. For example, in some embodiments, a modulation signal mayinclude a moderate amplitude and moderate duration, while in otherembodiments, a modulation signal may include a higher amplitude and ashorter duration. Various amplitudes and/or durations of field-inducingsignals across electrodes 158 a, 158 b may result in modulation signals,and whether a field-inducing signal rises to the level of a modulationsignal can depend on many factors (e.g., distance from a particularnerve to be stimulated; whether the nerve is branched; orientation ofthe induced electric field with respect to the nerve; type of tissuepresent between the electrodes and the nerve; etc.).

Whether a field inducing signal constitutes a modulation signal(resulting in an electric field that may cause nerve modulation) or asub-modulation signal (resulting in an electric field not intended tocause nerve modulation) may ultimately be controlled by processor 144 ofexternal unit 120. For example, in certain situations, processor 144 maydetermine that nerve modulation is appropriate. Under these conditions,processor 144 may cause signal source 144 and amplifier 146 to generatea modulation control signal on primary antenna 150 (i.e., a signalhaving a magnitude and/or duration selected such that a resultingsecondary signal on secondary antenna 152 will provide a modulationsignal at implant electrodes 158 a and 158 b).

Processor 144 may be configured to limit an amount of energy transferredfrom external unit 120 to implant unit 110. For example, in someembodiments, implant unit 110 may be associated with a threshold energylimit that may take into account multiple factors associated with thepatient and/or the implant. For example, in some cases, certain nervesof a patient should receive no more than a predetermined maximum amountof energy to minimize the risk of damaging the nerves and/or surroundingtissue. Additionally, circuitry 180 of implant unit 110 may includecomponents having a maximum operating voltage or power level that maycontribute to a practical threshold energy limit of implant unit 110.Processor 144 may be configured to account for such limitations whensetting the magnitude and/or duration of a primary signal to be appliedto primary antenna 150.

In addition to determining an upper limit of power that may be deliveredto implant unit 110, processor 144 may also determine a lower powerthreshold based, at least in part, on an efficacy of the deliveredpower. The lower power threshold may be computed based on a minimumamount of power that enables nerve modulation (e.g., signals havingpower levels above the lower power threshold may constitute modulationsignals while signals having power levels below the lower powerthreshold may constitute sub-modulation signals).

A lower power threshold may also be measured or provided in alternativeways. For example, appropriate circuitry or sensors in the implant unit110 may measure a lower power threshold. A lower power threshold may becomputed or sensed by an additional external device, and subsequentlyprogrammed into processor 144, or programmed into implant unit 110.Alternatively, implant unit 110 may be constructed with circuitry 180specifically chosen to generate signals at the electrodes of at leastthe lower power threshold. In still another embodiment, an antenna ofexternal unit 120 may be adjusted to accommodate or produce a signalcorresponding to a specific lower power threshold. The lower powerthreshold may vary from patient to patient, and may take into accountmultiple factors, such as, for example, modulation characteristics of aparticular patient's nerve fibers, a distance between implant unit 110and external unit 120 after implantation, and the size and configurationof implant unit components (e.g., antenna and implant electrodes), etc.

Processor 144 may also be configured to cause application ofsub-modulation control signals to primary antenna 150. Suchsub-modulation control signals may include an amplitude and/or durationthat result in a sub-modulation signal at electrodes 158 a, 158 b. Whilesuch sub-modulation control signals may not result in nerve modulation,such sub-modulation control signals may enable feedback-based control ofthe nerve modulation system. That is, in some embodiments, processor 144may be configured to cause application of a sub-modulation controlsignal to primary antenna 150. This signal may induce a secondary signalon secondary antenna 152, which, in turn, induces a primary coupledsignal component on primary antenna 150.

To analyze the primary coupled signal component induced on primaryantenna 150, external unit 120 may include a feedback circuit 148 (e.g.,a signal analyzer or detector, etc.), which may be placed in direct orindirect communication with primary antenna 150 and processor 144.Sub-modulation control signals may be applied to primary antenna 150 atany desired periodicity. In some embodiments, the sub-modulation controlsignals may be applied to primary antenna 150 at a rate of one everyfive seconds (or longer). In other embodiments, the sub-modulationcontrol signals may be applied more frequently (e.g., once every twoseconds, once per second, once per millisecond, once per nanosecond, ormultiple times per second). Further, it should be noted that feedbackmay also be received upon application of modulation control signals toprimary antenna 150 (i.e., those that result in nerve modulation), assuch modulation control signals may also result in generation of aprimary coupled signal component on primary antenna 150.

The primary coupled signal component may be fed to processor 144 byfeedback circuit 148 and may be used as a basis for determining a degreeof coupling between primary antenna 150 and secondary antenna 152. Thedegree of coupling may enable determination of the efficacy of theenergy transfer between two antennas. Processor 144 may also use thedetermined degree of coupling in regulating delivery of power to implantunit 110.

Processor 144 may be configured with any suitable logic for determininghow to regulate power transfer to implant unit 110 based on thedetermined degree of coupling. For example, where the primary coupledsignal component indicates that a degree of coupling has changed from abaseline coupling level, processor 144 may determine that secondaryantenna 152 has moved with respect to primary antenna 150 (either incoaxial offset, lateral offset, or angular offset, or any combination).Such movement, for example, may be associated with a movement of theimplant unit 110, and the tissue that it is associated with based on itsimplant location. Thus, in such situations, processor 144 may determinethat modulation of a nerve in the patient's body is appropriate. Moreparticularly, in response to an indication of a change in coupling,processor 144, in some embodiments, may cause application of amodulation control signal to primary antenna 150 in order to generate amodulation signal at implant electrodes 158 a, 158 b, e.g., to causemodulation of a nerve of the patient.

In an embodiment for the treatment of OSA, movement of an implant unit110 may be associated with movement of the tongue, which may indicatethe onset of a sleep apnea event or a sleep apnea precursor. The onsetof a sleep apnea event of sleep apnea precursor may require thestimulation of the genioglossus muscle of the patient to relieve oravert the event. Such stimulation may result in contraction of themuscle and movement of the patient's tongue away from the patient'sairway.

In embodiments for the treatment of head pain, including migraines,processor 144 may be configured to generate a modulation control signalbased on a signal from a user, for example, or a detected level ofneural activity in a sensory neuron (e.g. the greater occipital nerve ortrigeminal nerve) associated with head pain. A modulation control signalgenerated by the processor and applied to the primary antenna 150 maygenerate a modulation signal at implant electrodes 158 a, 158 b, e.g. tocause inhibition or blocking of a sensory nerve of the patient. Suchinhibition or blocking may decrease or eliminate the sensation of painfor the patient.

In embodiments for the treatment of hypertension, processor 144 may beconfigured to generate a modulation control signal based on, forexample, pre-programmed instructions and/or signals from an implantindicative of blood pressure. A modulation control signal generated bythe processor and applied to the primary antenna 150 may generate amodulation signal at implant electrodes 158 a, 158 b, e.g., to causeeither inhibition or stimulation of nerve of a patient, depending on therequirements. For example, a neuromodulator placed in a carotid arteryor jugular artery (i.e. in the vicinity of a carotid baroreceptor), mayreceive a modulation control signal tailored to induce a stimulationsignal at the electrodes, thereby causing the glossopharyngeal nerveassociated with the carotid baroreceptors to fire at an increased ratein order to signal the brain to lower blood pressure. Similar modulationof the glossopharyngeal nerve may be achieved with a neuromodulatorimplanted in a subcutaneous location in a patient's neck or behind apatient's ear. A neuromodulator place in a renal artery may receive amodulation control signal tailored to cause an inhibiting or blockingsignal at the electrodes, thereby inhibiting a signal to raise bloodpressure carried from the renal nerves to the kidneys.

Modulation control signals may include stimulation control signals, andsub-modulation control signals may include sub-stimulation controlsignals. Stimulation control signals may have any amplitude, pulseduration, or frequency combination that results in a stimulation signalat electrodes 158 a, 158 b. In some embodiments (e.g., at a frequency ofbetween about 6.5-13.6 MHz), stimulation control signals may include apulse duration of greater than about 50 microseconds and/or an amplitudeof approximately 0.5 amps, or between 0.1 amps and 1 amp, or between0.05 amps and 3 amps. Sub-stimulation control signals may have a pulseduration less than about 500, or less than about 200 nanoseconds and/oran amplitude less than about 1 amp, 0.5 amps, 0.1 amps, 0.05 amps, or0.01 amps. Of course, these values are meant to provide a generalreference only, as various combinations of values higher than or lowerthan the exemplary guidelines provided may or may not result in nervestimulation.

In some embodiments, stimulation control signals may include a pulsetrain, wherein each pulse includes a plurality of sub-pulses. Analternating current signal (e.g., at a frequency of between about6.5-13.6 MHz) may be used to generate the pulse train, as follows. Asub-pulse may have a duration of between 50-250 microseconds, or aduration of between 1 microsecond and 2 milliseconds, during which analternating current signal is turned on. For example, a 200 microsecondsub-pulse of a 10 MHz alternating current signal will includeapproximately 2000 periods. Each pulse may, in turn, have a duration ofbetween 100 and 500 milliseconds, during which sub-pulses occur at afrequency of between 25 and 100 Hz. For example, a 200 millisecond pulseof 50 Hz sub-pulses will include approximately 10 sub-pulses. Finally,in a pulse train, each pulse may be separated from the next by aduration of between 0.2 and 2 seconds. For example, in a pulse train of200 millisecond pulses, each separated by 1.3 seconds from the next, anew pulse will occur every 1.5 seconds. A pulse train of this embodimentmay be utilized, for example, to provide ongoing stimulation during atreatment session. In the context of OSA, a treatment session may be aperiod of time during which a subject is asleep and in need of treatmentto prevent OSA. Such a treatment session may last anywhere from aboutthree to ten hours. In the context of other conditions to which neuralmodulators of the present disclosure are applied, a treatment sessionmay be of varying length according to the duration of the treatedcondition.

As discussed above, a pulse train may be generated by a pulsed modeself-resonant transmitter as described herein. For example, eachsub-pulse may be generated by assembling multiple current loadingperiods and free oscillation periods. A period between sub-pulses mayinclude an energy accumulation period, after which another sub-pulsecomprising multiple current loading periods and free oscillation periodsmay be generated. In this manner, a pulse train having any or all of theparameters as described above may be generated utilizing a pulsed modeself-resonant transmitter.

Processor 144 may be configured to determine a degree of couplingbetween primary antenna 150 and secondary antenna 152 by monitoring oneor more aspects of the primary coupled signal component received throughfeedback circuit 148. In some embodiments, processor 144 may determine adegree of coupling between primary antenna 150 and secondary antenna 152by monitoring a voltage level associated with the primary coupled signalcomponent, a current level, or any other attribute that may depend onthe degree of coupling between primary antenna 150 and secondary antenna152. For example, in response to periodic sub-modulation signals appliedto primary antenna 150, processor 144 may determine a baseline voltagelevel or current level associated with the primary coupled signalcomponent. This baseline voltage level, for example, may be associatedwith a range of movement of the patient's tongue when a sleep apneaevent or its precursor is not occurring, e.g. during normal breathing.As the patient's tongue moves toward a position associated with a sleepapnea event or its precursor, the coaxial, lateral, or angular offsetbetween primary antenna 150 and secondary antenna 152 may change. As aresult, the degree of coupling between primary antenna 150 and secondaryantenna 152 may change, and the voltage level or current level of theprimary coupled signal component on primary antenna 150 may also change.Processor 144 may be configured to recognize a sleep apnea event or itsprecursor when a voltage level, current level, or other electricalcharacteristic associated with the primary coupled signal componentchanges by a predetermined amount or reaches a predetermined absolutevalue.

FIG. 9 provides a graph that illustrates this principle in more detail.For a two-coil system where one coil receives a radio frequency (RF)drive signal, graph 200 plots a rate of change in induced current in thereceiving coil as a function of coaxial distance between the coils. Forvarious coil diameters and initial displacements, graph 200 illustratesthe sensitivity of the induced current to further displacement betweenthe coils, moving them either closer together or further apart. It alsoindicates that, overall, the induced current in the secondary coil willdecrease as the secondary coil is moved away from the primary, drivecoil, i.e. the rate of change of induced current, in mA/mm, isconsistently negative. The sensitivity of the induced current to furtherdisplacement between the coils varies with distance. For example, at aseparation distance of 10 mm, the rate of change in current as afunction of additional displacement in a 14 mm coil is approximately −6mA/mm. If the displacement of the coils is approximately 22 mm, the rateof change in the induced current in response to additional displacementis approximately −11 mA/mm, which corresponds to a local maximum in therate of change of the induced current. Increasing the separationdistance beyond 22 mm continues to result in a decline in the inducedcurrent in the secondary coil, but the rate of change decreases. Forexample, at a separation distance of about 30 mm, the 14 mm coilexperiences a rate of change in the induced current in response toadditional displacement of about −8 mA/mm. With this type ofinformation, processor 144 may be able to determine a particular degreeof coupling between primary antenna 150 and secondary antenna 152, atany given time, by observing the magnitude and/or rate of change in themagnitude of the current associated with the primary coupled signalcomponent on primary antenna 150.

In some embodiments, an initially detected coupling degree may establisha baseline range when the patient attaches external unit 120 to theskin. Presumably, while the patient is awake, the tongue is not blockingthe patient's airway and moves with the patients breathing in a naturalrange, where coupling between primary antenna 150 and secondary antenna152 may be within a baseline range. A baseline coupling range mayencompass a maximum coupling between primary antenna 150 and secondaryantenna 152. A baseline coupling range may also encompass a range thatdoes not include a maximum coupling level between primary antenna 150and secondary antenna 152. Thus, the initially determined coupling maybe fairly representative of a non-sleep apnea condition and may be usedby processor 144 as a baseline in determining a degree of couplingbetween primary antenna 150 and secondary antenna 152.

As the patient wears external unit 120, processor 144 may periodicallyscan over a range of primary signal amplitudes to determine currentvalues of coupling. If a periodic scan results in determination of adegree of coupling different from the baseline coupling, processor 144may determine that there has been a change from the baseline initialconditions.

By periodically determining a degree of coupling value, processor 144may be configured to determine, in situ, appropriate parameter valuesfor the modulation control signal that will ultimately result in nervemodulation. For example, by determining the degree of coupling betweenprimary antenna 150 and secondary antenna 152, processor 144 may beconfigured to select characteristics of the modulation control signal(e.g., amplitude, pulse duration, frequency, etc.) that may provide amodulation signal at electrodes 158 a, 158 b in proportion to orotherwise related to the determined degree of coupling. In someembodiments, processor 144 may access a lookup table or other datastored in a memory correlating modulation control signal parametervalues with degree of coupling. In this way, processor 144 may adjustthe applied modulation control signal in response to an observed degreeof coupling.

Additionally or alternatively, processor 144 may be configured todetermine the degree of coupling between primary antenna 150 andsecondary antenna 152 during modulation. The tongue, or other structureon or near which the implant is located, and thus implant unit 110, maymove as a result of modulation. Thus, the degree of coupling may changeduring modulation. Processor 144 may be configured to determine thedegree of coupling as it changes during modulation, in order todynamically adjust characteristics of the modulation control signalaccording to the changing degree of coupling. This adjustment may permitprocessor 144 to cause implant unit 110 to provide an appropriatemodulation signal at electrodes 158 a, 158 b throughout a modulationevent. For example, processor 144 may alter the primary signal inaccordance with the changing degree of coupling in order to maintain aconstant modulation signal, or to cause the modulation signal to bereduced in a controlled manner according to patient needs.

More particularly, the response of processor 144 may be correlated tothe determined degree of coupling. In situations where processor 144determines that the degree of coupling between primary antenna 150 andsecondary antenna has fallen only slightly below a predeterminedcoupling threshold (e.g. during snoring or during a small vibration ofthe tongue or other sleep apnea event precursor), processor 144 maydetermine that only a small response is necessary. Thus, processor 144may select modulation control signal parameters that will result in arelatively small response (e.g., a short stimulation of a nerve, smallmuscle contraction, etc.). Where, however, processor 144 determines thatthe degree of coupling has fallen substantially below the predeterminedcoupling threshold (e.g., where the tongue has moved enough to cause asleep apnea event), processor 144 may determine that a larger responseis required. As a result, processor 144 may select modulation controlsignal parameters that will result in a larger response. In someembodiments, only enough power may be transmitted to implant unit 110 tocause the desired level of response. In other words, processor 144 maybe configured to cause a metered response based on the determined degreeof coupling between primary antenna 150 and secondary antenna 152. Asthe determined degree of coupling decreases, processor 144 may causetransfer of power in increasing amounts. Such an approach may preservebattery life in the external unit 120, may protect circuitry 170 andcircuitry 180, may increase effectiveness in addressing the type ofdetected condition (e.g., sleep apnea, snoring, tongue movement, etc.),and may be more comfortable for the patient.

In some embodiments, processor 144 may employ an iterative process inorder to select modulation control signal parameters that result in adesired response level. For example, upon determining that a modulationcontrol signal should be generated, processor 144 may cause generationof an initial modulation control signal based on a set of predeterminedparameter values. If feedback from feedback circuit 148 indicates that anerve has been modulated (e.g. if an increase in a degree of coupling isobserved), then processor 144 may return to a monitoring mode by issuingsub-modulation control signals. If, on the other hand, the feedbacksuggests that the intended nerve modulation did not occur as a result ofthe intended modulation control signal or that modulation of the nerveoccurred but only partially provided the desired result (e.g. movementof the tongue only partially away from the airway), processor 144 maychange one or more parameter values associated with the modulationcontrol signal (e.g., the amplitude, pulse duration, etc.).

Where no nerve modulation occurred, processor 144 may increase one ormore parameters of the modulation control signal periodically until thefeedback indicates that nerve modulation has occurred. Where nervemodulation occurred, but did not produce the desired result, processor144 may re-evaluate the degree of coupling between primary antenna 150and secondary antenna 152 and select new parameters for the modulationcontrol signal targeted toward achieving a desired result. For example,where stimulation of a nerve causes the tongue to move only partiallyaway from the patient's airway, additional stimulation may be desired.Because the tongue has moved away from the airway, however, implant unit110 may be closer to external unit 120 and, therefore, the degree ofcoupling may have increased. As a result, to move the tongue a remainingdistance to a desired location may require transfer to implant unit 110of a smaller amount of power than what was supplied prior to the laststimulation-induced movement of the tongue. Thus, based on a newlydetermined degree of coupling, processor 144 can select new parametersfor the stimulation control signal aimed at moving the tongue theremaining distance to the desired location.

In one mode of operation, processor 144 may be configured to sweep overa range of parameter values until nerve modulation is achieved. Forexample, in circumstances where an applied sub-modulation control signalresults in feedback indicating that nerve modulation is appropriate,processor 144 may use the last applied sub-modulation control signal asa starting point for generation of the modulation control signal. Theamplitude and/or pulse duration (or other parameters) associated withthe signal applied to primary antenna 150 may be iteratively increasedby predetermined amounts and at a predetermined rate until the feedbackindicates that nerve modulation has occurred.

Processor 144 may be configured to determine or derive variousphysiologic data based on the determined degree of coupling betweenprimary antenna 150 and secondary antenna 152. For example, in someembodiments the degree of coupling may indicate a distance betweenexternal unit 120 and implant unit 110, which processor 144 may use todetermine a position of external unit 120 or a relative position of apatient's tongue. Monitoring the degree of coupling can also providesuch physiologic data as whether a patient's tongue is moving orvibrating (e.g. whether the patient is snoring), by how much the tongueis moving or vibrating, the direction of motion of the tongue, the rateof motion of the tongue, etc.

In response to any of these determined physiologic data, processor 144may regulate delivery of power to implant unit 110 based on thedetermined physiologic data. For example, processor 144 may selectparameters for a particular modulation control signal or series ofmodulation control signals for addressing a specific condition relatingto the determined physiologic data. If the physiologic data indicatesthat the tongue is vibrating, for example, processor 144 may determinethat a sleep apnea event is likely to occur and may issue a response bydelivering power to implant unit 110 in an amount selected to addressthe particular situation. If the tongue is in a position blocking thepatient's airway (or partially blocking a patient's airway), but thephysiologic data indicates that the tongue is moving away from theairway, processor 144 may opt to not deliver power and wait to determineif the tongue clears on its own. Alternatively, processor 144 maydeliver a small amount of power to implant unit 110 (e.g., especiallywhere a determined rate of movement indicates that the tongue is movingslowly away from the patient's airway) to encourage the tongue tocontinue moving away from the patient's airway or to speed itsprogression away from the airway. The scenarios described are exemplaryonly. Processor 144 may be configured with software and/or logicenabling it to address a variety of different physiologic scenarios withparticularity. In each case, processor 144 may be configured to use thephysiologic data to determine an amount of power to be delivered toimplant unit 110 in order to modulate nerves associated with the tonguewith the appropriate amount of energy.

The disclosed embodiments may be used in conjunction with a method forregulating delivery of power to an implant unit. The method may includedetermining a degree of coupling between primary antenna 150 associatedwith external unit 120 and secondary antenna 152 associated with implantunit 110, implanted in the body of a patient. Determining the degree ofcoupling may be accomplished by processor 144 located external toimplant unit 110 and that may be associated with external unit 120.Processor 144 may be configured to regulate delivery of power from theexternal unit to the implant unit based on the determined degree ofcoupling.

As previously discussed, the degree of coupling determination may enablethe processor to further determine a location of the implant unit. Themotion of the implant unit may correspond to motion of the body partwhere the implant unit may be attached. This may be consideredphysiologic data received by the processor. The processor may,accordingly, be configured to regulate delivery of power from the powersource to the implant unit based on the physiologic data. In alternativeembodiments, the degree of coupling determination may enable theprocessor to determine information pertaining to a condition of theimplant unit. Such a condition may include location as well asinformation pertaining to an internal state of the implant unit. Theprocessor may, according to the condition of the implant unit, beconfigured to regulate delivery of power from the power source to theimplant unit based on the condition data.

In some embodiments, implant unit 110 may include a processor located onthe implant. A processor located on implant unit 110 may perform all orsome of the processes described with respect to the at least oneprocessor associated with an external unit. For example, a processorassociated with implant unit 110 may be configured to receive a controlsignal prompting the implant controller to turn on and cause amodulation signal to be applied to the implant electrodes for modulatinga nerve. Such a processor may also be configured to monitor varioussensors associated with the implant unit and to transmit thisinformation back to and external unit. Power for the processor unit maybe supplied by an onboard power source or received via transmissionsfrom an external unit.

In other embodiments, implant unit 110 may be self-sufficient, includingits own power source and a processor configured to operate the implantunit 110 with no external interaction. For example, with a suitablepower source, the processor of implant unit 110 could be configured tomonitor conditions in the body of a subject (via one or more sensors orother means), determining when those conditions warrant modulation of anerve, and generate a signal to the electrodes to modulate a nerve. Thepower source could be regenerative based on movement or biologicalfunction; or the power sources could be periodically rechargeable froman external location, such as, for example, through induction.

Other embodiments of the present disclosure will be apparent to thoseskilled in the art from consideration of the specification and practiceof the present disclosure.

Additional aspects of the invention are described in the followingnumbered paragraphs, which are part of the description of exemplaryembodiments of the invention. Each numbered paragraph stands on its ownas a separate embodiment of the invention.

What is claimed is:
 1. A device for treating sleep apnea by powering animplant within a body of a subject from a location external to thesubject, wherein the implant requires a threshold rate of power increasein order to operate in at least one stimulate a hypoglossal nerve andthereby cause contraction of a genioglossus muscle, the devicecomprising: an antenna configured for location between a neck and a chinof the subject and to wirelessly transmit energy to the implant, theantenna being part of a resonant circuit; a power storage unitconfigured to store energy from a power source incapable of deliveringthe threshold rate of power increase to enable the implant unit tooperate in the at least one mode; and a power release unit configured torelease a pulse of energy from the power storage unit to the antenna,the pulse of energy being sufficient to enable the implant unit tostimulate the hypoglossal nerve to cause contraction of the genioglossusmuscle, and wherein the resonant circuit is configured to generate asignal for transmission to the implant by electrical oscillation in theresonant circuit at a resonant frequency of the resonant circuit whenthe pulse of energy is received by the resonant circuit.
 2. The deviceof claim 1, further comprising a substrate, wherein the antenna isassociated with the substrate.
 3. The device of claim 2, wherein thesubstrate and the antenna are flexible.
 4. The device of claim 1,further comprising at least one processor configured to control a modeof operation of the power release unit such that in a first mode ofoperation the power storage unit receives power from the power source,and in a second mode of operation, the power storage unit releases powerto the antenna to cause an oscillation in the primary antenna at aresonance frequency.
 5. The device of claim 1, wherein the antenna isconfigured to be located external to a subject's body.
 6. The device ofclaim 1, wherein the power source is configured to generate electricalenergy.
 7. The device of claim 1, wherein the power storage unit isconfigured to store electrical energy generated by the power source. 8.The device of claim 1, wherein the power release unit is configured tocontrol electrical energy transfer between the power source and thepower storage unit and between the power storage unit and the antenna,wherein at least the antenna is part of an electrical circuit having aresonance frequency.